1. Field of the Invention
The present invention relates to a peel apart type instant film unit.
2. Description of the Prior Art
As is well known in the art, the peel apart type instant film unit shown and illustrated in detail, for example, in Japanese Patent Publication No. 44-2528 (which is hereinafter referred to as a film unit) mainly comprises a photosensitive sheet having a photosensitive emulsion layer capable of having a developable latent image formed therein and an image receiving sheet adapted to provide a support for a positive transfer image produced, for example, by a diffusion-transfer reversal process. The photosensitive sheet is coupled to a carrier sheet and the image receiving sheet is mounted on a mask having a leading end connected to the carrier sheet.
After exposure, the film unit is withdrawn out of a film cassette or pack for superposing the sheets by drawing them through a pair of pressure applying rollers. At this time, a pod mounted on the carrier sheet is compressively ruptured for releasing a jellied processing liquid contained therein. The pressure applying rollers are adapted to distribute the processing liquid between the superposed sheets as the film unit is advanced therebetween for spreading the processing liquid in a thin, uniform layer. As is well known in the art of instant photography, upon the withdrawal of the film unit from a camera, the diffusion transfer process takes place exteriorly of the camera in a certain time. The sheets are then peeled apart form each other to reveal a desired reverse image in the image receiving sheet.
The mask, which is conventially made of any of the flexible sheet materials including kraft paper, glassine paper and the like, is provided with a weak sealing layer coated on the back surface thereof. The mask member is heat sealed to the image receiving sheet through the weak sealing layer. A conventional weak sealing layer comprises a separating layer coated on the back surface of the mask and an adhesive layer coated on the separating layer, which weak sealing layer is disclosed in U.S. Pat. Nos. 2,563,387, 2,659,673, and 2,678,274. In the above-mentioned U.S. Pat. No. 2,563,387, the material of the separating layer is zein of vegetable albumin with a triethylene glycol mixed as a plasticizer, as the adhesive agent is an ethylcellulose mixed with diethylphtalate as plasticizer. In the U.S. Pat. Nos. 2,659,673 and 2,678,274, polyvinyl alcohol is used for the separating agent and ethylcellulose mixed with either diethylphthalate or triphenyl phosphate as plasticizer is used for the adhesive agent. The adhesive strength between the mask and the separating layer and between the adhesive layer and the image receiving layer is intended to be greater than that between the separating layer and the adhesive layer for producing an easy and fine separation between the layers when the image receiving sheet is peeled apart from the mask in order to transfer the adhesive layer onto the image receiving sheet and thereby provide a frame with white color for defining an image area in the image receiving sheet.
One of problems in the conventional weak sealing layer is that parts of the adhesive layer are not transferred to the image receiving sheet and that the separating layer is partially exfoliated from the mask and clings to the image receiving sheet as well as the adhesive layer when the image receiving sheet is peeled apart from the mask after a predetermined separation time (which substantially equals to processing time) which is about 30 seconds in the case of a monochromatic film unit. As a result, the frame becomes dappled and fouled externally. It was recognized that the dapple mark becomes more severe as the separation time becomes longer. Furthermore, dapple marks appear at the peripheral region of the image area and harm image quality.